Soybean (Glycine max) is one of the most important agricultural crops, with an annual crop yield of more than 200 million metric tons, and an estimated value exceeding 40 billion U.S. dollars worldwide. Soybean accounts for over 97% of all oilseed production globally. Thus, reliable and efficient methods for improving the quality and yield of this valuable crop are of significant interest.
Traditional breeding methods for improving soybean have been constrained because the majority of soybean cultivars are derived from only a few parental lines, leading to a narrow germplasm base for breeding. Christou et al., TIBTECH 8:145-151 (1990). Modern research efforts have focused on plant genetic engineering techniques to improve soybean production. Transgenic methods are designed to introduce desired genes into the heritable germline of crop plants to generate elite plant lines. The approach has successfully increased the resistance of several other crop plants to disease, insects, and herbicides, while improving nutritional value.
Several methods have been developed for transferring genes into plant tissue, including high velocity microprojection, microinjection, electroporation, and direct DNA uptake. Agrobacterium-mediated gene transformation has more recently been used to introduce genes of interest into soybeans. However, soybeans have proven to be a challenging system for transgenic engineering. Efficient transformation and regeneration of soybean explants is difficult to achieve, and frequently hard to repeat.
Agrobacterium tumefaciens, a pathogenic, soil-dwelling bacterium, has the inherent ability to transfer its DNA, called T-DNA, into host plant cells and to induce the host cells to produce metabolites useful for bacterial nutrition. Using recombinant techniques, some or all of the T-DNA may be replaced with a gene or genes of interest, creating a bacterial vector useful for transforming the host plant. Agrobacterium-mediated gene transfer is typically directed at undifferentiated cells in tissue culture, but may also be directed at differentiated cells taken from the leaf or stem of the plant. A number of procedures have been developed for Agrobacterium-mediated transformation of soybean, which may loosely be classified based on the explant tissue subjected to transformation.
U.S. Pat. No. 7,696,408, Olhoft, et al., discloses a cotyledonary node method for transforming both monocotyledonous and dicotyledonous plants. The “cot node” method involves removing the hypocotyl from 5-7 day old soybean seedlings by cutting just below the cotyledonary node, splitting and separating the remaining hypocotyl segment with the cotyledons, and removing the epicotyl from the cotyledon. The cotyledonary explant is wounded in the region of the axillary bud and/or cotyledonary node, and cultivated with Agrobacterium tumefaciens for five days in the dark. The method requires in-vitro germination of the seeds, and the wounding step introduces significant variability.
U.S. Pat. No. 6,384,301, Martinelli et al., discloses Agrobacterium-mediated gene delivery into living meristem tissue from soybean embryos excised from soybean seeds, followed by culturing of the meristem explant with a selection agent and hormone to induce shoot formation. Like the “cot node” method, the meristem explants are preferably wounded prior to infection.
U.S. Pat. No. 7,473,822, Paz et al., discloses a modified cotyledonary node method called the “half-seed explant” method. Mature soybean seeds are imbibed, surface-sterilized and split along the hilum. Prior to infection, the embryonic axis and shoots are completely removed, but no other wounding occurs. Agrobacterium-mediated transformation proceeds, potential transformants are selected, and explants are regenerated on selection medium.
Transformation efficiencies remain relatively low with these methods, on the order of 0.3% to 2.8% for the “cot node” method, 1.2 to 4.7% for the “meristem explant” method, and between 3.2% and 8.7% (overall 4.9%) for the “half-seed explant” method. Transformation efficiencies of approximately 3% are typical in the art.
An improved “split-seed” transgenic protocol may accelerate future production and development of transgenic soybean products. An efficient and high-throughput method for stable integration of a transgene into soybean tissue would facilitate breeding programs and have the potential to increase crop productivity.